The present invention relates to a secondary hardening type high temperature wear-resistant sintered alloy, and more specifically to a secondary hardening type high temperature wear-resistant sintered alloy which has no only excellent wear resistance, heat resistance, strength and corrosion resistance, but also has a good workability (or working characteristic) and may suitably be used for a material for forming a valve seat to be used for an internal combustion engine, for example.
In general, a secondary hardening type sintered alloy which is capable of having increased the hardness or strength on the basis of a pressure or a thermal load which is to be applied thereto after the working thereof, has been used for tool steel. In addition, the secondary hardening type sintered alloy may suitably be used as a material constituting a valve seat to be used for an internal combustion engine. Particularly, various investigations have been made as to the possibility thereof of such material for the valve seat to be used for an internal combustion engine.
On the other hand, the environment in which the valve seat for the internal combustion engine is to be used has steadily become severe along with an improvement in the performance of the engine. In Order to attain an engine which has plural valves (i.e., multi valve engine), which is capable of effecting combustion in a dilute phase at a high temperature, and which is capable of rotating at a high speed, it is necessary to improve the characteristics of the valve seat such as the wear resistance, heat resistance and strength.
Hitherto, there has generally been used an iron type sintered alloy as the material for forming the valve seat for the internal combustion engine. In order to improve the characteristics of the valve seat for the internal combustion engine which is formed of such a conventional iron type sintered alloy, various investigations have been made.
For example, in an attempt to increase wear resistance of known iron type sintered alloys, hard particles comprising a Stellite type alloy, Eatnite type alloy, and various ceramics (e.g., carbides, oxides, nitrides, etc.) have been added thereto, a solid lubricating agent such as Pb, Pb alloy, graphite, fluoride, and sulfide have been added or infiltrated thereto, an oxide layer (or film) has been formed, on a surface thereof, and such iron type alloys which have been treated with steam, etc. Particularly, there has widely been used the iron type to which the hard particles as described above have been added.
In addition, in an attempt to improve heat resistance of known iron type alloys wherein the pores thereof have been sealed by use of Cu or a Cu alloy, and such iron type alloys have been subjected to forging, repressing, etc., so that the true density thereof is increased or it is densified. Also an alloy element such as Co, Ni and P have been added to such iron type alloys.
In addition, in an attempt to improve strength of such iron type, such alloys have been subjected to the same treatment as that for the above improvement in the heat resistance, and have been heat treated after the attempted improvement in wear resistance and heat resistance as described above.
In the iron type alloy as described above, however, by attempting to improve wear resistance (e.g., by increasing the amount of the above hard particles to be added thereto), the workability (or cuttability) thereof is decreased, and further, the compression molding property and the sintering property are deteriorated, whereby the strength of the sintered product is decreased. In such a case, when the resultant iron type alloy is used as a valve seat for an internal combustion engine, the valve to be used in combination therewith is liable to be worn. In addition, by attempting to improve wear resistance by adding or infiltrating a solid lubricating agent to the alloy, there is posed a problem such that the strength of the alloy is decreased. Further, by attempting to improve resistance by the formation of the oxide layer or by steam treatment, there is posed a problem such that the strength and tenacity thereof are decreased. Furthermore, in the conventional iron type alloy, the wear resistance, heat resistance and strength are intended to be improved simultaneously, the number of the steps constituting such a production process is increased and the amount or number of the materials to be used for such a production process is increased. As a result, there is posed a problem such that the production cost of such an alloy is raised.
On the other hand, there have been developed various engines which are capable of using a gasoline alternate fuel (i.e., a fuel which is usable for an engine in place of gasoline) on the basis of the demands such as the protection of the earth environment and the reduction in the amount of crude oil to be consumed. Among such engines, in the case of an engine using an alcohol as a fuel, since corrosion based on formic acid produced in the cylinder thereof accelerates or promotes the wear of the valve seat, the material for constituting the valve seat is required to have a sufficient corrosion resistance. However, the valve seat for an internal combustion engine which has been formed of a conventional material, does not have a sufficient corrosion resistance required for the alcohol engine in addition to the performances required for the conventional engine.
Accordingly, a material having improved characteristics such as wear resistance, heat resistance strength, and corrosion resistance while maintaining good workability, has been desired.